Serial links are used in many applications, for example, optical communications. As depicted in FIG. 1, a prior art serial sink 100 comprises a transmitter 110, a transmission medium 120, and a receiver 130. The transmitter 110 transmits onto a first end 121 of the transmission medium 120 a first signal S1 using a two-level signaling scheme to represent a first serial binary data stream D1 timed in accordance with a first clock CLK1. The first signal S1 traverses along the transmission medium 120 and evolves into a second signal S2 as it reaches at a second end 122 of the transmission medium 120. The second signal S2, which is a continuous-time signal, is received by the receiver 130 at the second end 122 of the transmission medium 120. The receiver comprises: an equalizer 132 for receiving the second signal S2 and outputting a third signal S3, and a CDR (clock data recovery) apparatus 131 for generating a second clock CLK2 by extracting a timing embedded in third signal S3 and for using the second clock CLK 2 to sample the second signal S3 to generate a second serial binary data stream D2. When the CDR apparatus 131 functions correctly, the second serial binary data stream D2 will substantially match the first serial binary D1, except for a delay. The purpose of equalizer 132 is to correct a distortion of the second signal S2 due to dispersion of the transmission medium 120.
An exemplary waveform of the first signal S1, the second signal S2, and the third signal S3 are shown in FIG. 2. The first signal S1 is a typical NRZ (non-return-to-zero) waveform that is either of a first level (1) or a second level (−1), representing either a binary “1” or a binary “0” data, respectively. Due to distortion of the transmission medium 120, the second signal S2 is distorted and thus quite different from the first signal; in particular, it fails to reach the full level when the first signal S1 undergoes a consecutive sign change, as illustrated by the difference in waveform between 201 and 202. The equalizer 132 is used to correct the distortion, so that the third signal S3 can be more like the first signal S1, as illustrated by the similarity between 203 and 201. Equalizer 132, however, is an analog circuit that is susceptible to variation due to manufacturing process, temperature, and supply voltage of the circuit.
What is disclosed is a method and apparatus of a digital equalizer for correcting a distortion due to dispersion of the transmission medium.